Lost in Secular Evolution: The Case of a Low-Mass Classical Bulge

Abstract

The existence of a classical bulge in disk galaxies holds important clue to the assembly history of galaxies. Finding observational evidence of very low mass classical bulges particularly in barred galaxies including our Milky Way, is a challenging task as the bar driven secular evolution might bring significant dynamical change to these bulges alongside the stellar disk.Using high-resolution N-body simulation, we show that if a cool stellar disk is assembled around a non-rotating low-mass classical bulge, the disk rapidly grows a strong bar within a few rotation time scales. Later, the bar driven secular process transform the initial classical bulge into a flattened rotating stellar system whose central part also have grown a bar-like component rotating in sync with the disk bar. During this time, a boxy/peanut (hereafter, B/P) bulge is formed via the buckling instability of the disk bar and the vertical extent of this B/P bulge being slightly higher than that of the classical bulge, it encompasses the whole classical bulge. The resulting composite bulge appears to be both photometrically and kinematically identical to a B/P bulge without any obvious signature of the classical component. Our analysis suggest that many barred galaxies in the local universe might be hiding such low-mass classical bulges. We suggest that stellar population and chemodynamical analysis might be required in establishing the evidence for such low-mass classical bulges.

The prevalence of pseudo-bulges in the Auriga simulations

“…Bulges formed by mergers can change their shape dur-ing their evolution. It is possible that the formation of a bar on top of a small classical bulge formed at high redshift can dynamically affect the non-rotating component, making it indistinguishable from a pseudo-bulge at z = 0, as is demonstrated by simulations (Saha et al 2012;Saha 2015) and as can be seen clearly in the case of Au2 in Fig. 6.…”

SUPERMASSIVE BLACK HOLES AND THEIR HOST SPHEROIDS. II. THE RED AND BLUE SEQUENCE IN THE MBH–M*,SPH DIAGRAM

“…Pseudo-bulges are notoriously hard to identify (Graham 2013(Graham , 2014(Graham , 2015(Graham , 2016. For example, mergers can create bulges that rotate (e.g., Bekki 2010; Keselman & Nusser 2012), and bars can spin-up classical bulges (e.g., Saha et al 2012;Saha 2015), thus rotation is not a definitive signature of a pseudo-bulge. Furthermore, many galaxies host both a classical and a pseudo-bulge (e.g., Erwin et al 2003Erwin et al , 2015Athanassoula 2005;Gadotti 2009;MacArthur et al 2009;Erwin 2010;dos Anjos & da Silva 2013;Seidel et al 2015).…”

Relative growth of black holes and the stellar components of galaxies

“…2.2). However, some authors argue that the local frequency of spirals with low values of the B/T ratio (B/T < ∼ 0.2) may indicate that secular processes can appreciably affect the formation of classical bulges (see, e.g., Weinzirl et al 2009;Saha 2015), a possibility explored in some theoretical works based on SAMs (see Perry et al 2009;De Lucia et al 2011; see also Brenman et al 2015). The effect of assuming such a hypothesis can be explored in our model by assuming that all bulges are formed through the disc instability process (case iii in Sect.…”